1 . Field of the Invention
Embodiments of the present invention relate to an organic light-emitting display device, and more particularly, to a thin film transistor of an organic light-emitting device. Embodiments of the present invention are suitable for a wide scope of applications. In particular, embodiments of the present invention are suitable for driving a thin film transistor of an organic light-emitting display device for a long time period, and a method for fabricating the driving thin film transistor.
2 . Discussion of the Related Art
Recently, various flat displays have been developed, which can overcome the shortcomings of cathode ray tubes (CRT), which are heavy and bulky. Examples of these flat display devices are a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light-emitting display (LED).
In particular, the LED is a self light-emitting device in which a fluorescent substance emits light by coupling electrons and holes. The LED may be divided into an inorganic light-emitting display device which uses a fluorescent substance of an inorganic compound; and an organic light-emitting display device which uses a fluorescent substance of an organic compound. The LED is of particular interest because it requires a low-driving voltage, is self light-emitting and thin, and provides a wide viewing angle, a fast response and a high contrast.
The organic LED device includes an electron injection layer, an electron transport layer, a light-emission layer, a hole transport layer, and a hole injection layer formed between a cathode and an anode. If a predetermined voltage is applied between the cathode and the anode, an electron generated in the cathode moves to the light-emission layer through the electron injection layer and the electron transport layer. Also, a hole generated in the anode moves to the light-emission layer through the hole injection layer and the hole transport layer. In the light-emission layer, the electron supplied by the electron transport layer is re-coupled with the hole supplied by the hole transport layer, thereby emitting the light.
FIG. 1A shows a circuit diagram illustrating one pixel of the related art organic light-emitting display device. Referring to FIG. 1A, the pixel in the related art organic light-emitting display device includes a gate line (not shown, Vdd-applying side) and a data line (not shown, Idata-applying side) crossing each other; first and second switching transistors T1 and T2; a third switching transistor T3; and a fourth switching transistor T4. The first and second transistors T1 and T2 include gate terminals connected to the gate line, and drain terminals connected to the data line. Also, the third switching transistor T3 is connected between a source terminal of the first switching transistor T1 and a source terminal of the second switching transistor T2. The fourth switching transistor T4 is connected between a power-supplying terminal (VDD) and the source terminal of the first switching transistor T1. Further, an organic light-emitting diode (OLED) is formed between a ground terminal and source terminals of the third and fourth switching transistors T3 and T4. Then, a storage capacitor Cs is formed between a gate terminal of the third switching transistor T3 and the ground terminal. In this case, the third switching transistor T3 functions as a driving transistor to drive the OLED.
Operation of the related art organic light-emitting display device will be described as follows. If an address voltage VADD is turned-on (on), the first and second switching transistors T1 and T2 are turned-on. Then, a data current Idata is charged on the storage capacitor Cs through the first switching transistor T1. Thus, the third switching transistor T3 is turned-on. Hence, a current flows in the OLED through the second and third switching transistors T2 and T3, and the fourth switching transistor T4 is also concurrently turned-on. Thus, electric charges supplied from the address voltage VADD flow into the fourth switching transistor T4. If the address voltage VADD is turned-off, the second switching transistor T2 is turned-off. However, the third and fourth switching transistors T3 and T4 are maintained in the turning-on state by the voltage stored in the storage capacitor (Cs), whereby the current flows in.
Thus, it is necessary that the third and fourth switching transistors T3 and T4 have the same threshold voltage Vth. That is, the threshold voltage of the fourth switching transistor T4 should be compensated by the third switching transistor T3. However, since the voltage applied to the drain terminal of the third switching transistor T3 is different from that of the fourth switching transistor T4, the threshold voltage of the third switching transistor T3 is different from the threshold voltage of the fourth switching transistor T4, thereby causing an incorrect compensation. During current compensation, it takes a long time to charge the storage capacitor (Cs) in the low current level.
FIG. 1B shows a graphical illustration of a variation of a drain current in accordance with a data voltage in the driving thin film transistor of FIG. 1A. Referring to FIG. 1B, the driving thin film transistor T3 has a threshold voltage Vth1 greater than or equal 0. A gate bias voltage of the driving TFT T3 changes in accordance with a data voltage Vdata within a range from Vd_min to Vd_max. A positive stress (+stress) is applied to the driving thin film transistor. Thus, the threshold voltage changes with time. Hence, the life span of the driving thin film transistor T3 becomes shorter.
For example, if the driving thin film transistor T3 is driven in the predetermined range Vd_min to Vdmax of the data voltage range Vdata range, an initial data voltage value Vd_min of a minimum value corresponds to an initial threshold voltage Vth1 of the driving thin film transistor. For example, Vth1 can be 0V or slightly larger value than 0V. However, if the positive bias stress is continuously applied to the driving thin film transistor T3, the threshold voltage increases. As a result, the driving thin film transistor is not turned-on due to the increase of threshold voltage after a predetermined time period.
In the related art organic light-emitting display device, four switching transistors are provided for one pixel. Thus, the portion provided with the switching elements and the storage capacitor serves as light-shielding portions, whereby the resolution and aperture ratio deteriorates, thereby lowering the picture quality.
FIG. 2 shows a cross-sectional view of the driving thin film transistor of FIG. 1A. Referring to FIG. 2, the driving thin film transistor T3 of FIG. 1A includes a gate electrode 11 formed on a predetermined portion of a substrate 10; a gate insulation layer 13 formed on an entire surface of the substrate 10 including the gate electrode 11; a semiconductor layer 15 formed on the gate insulation layer 13 and formed in an island shape of covering the gate electrode 11; and source and drain electrodes 17 and 18 formed at both sides of the semiconductor layer 15.
Also, a passivation layer 16 is formed on the entire surface of the substrate 10 including the source and drain electrodes 17 and 18. The passivation layer 16 has a contact hole to expose the upper side of the drain electrode 18. A first transparent electrode 19 is electrically connected to the drain electrode 18 by filling-up the contact hole with a transparent material. Then, an organic light-emission layer is interposed between the first transparent electrode 19 and a second electrode (not shown), whereby images are displayed based on the operation of the driving thin film transistor. The first electrode 19, the second electrode, and the organic light-emission layer function as the OLED.
The semiconductor layer 15 includes an amorphous silicon layer 13, and an impurity layer 14 formed under the source and drain electrodes 17 and 18. If the driving thin film transistor is provided with the semiconductor layer 15 including the amorphous silicon layer 13, a channel region is defined in the amorphous silicon layer 13. Such a structure is referred to as a back channel etched (BCE) type amorphous silicon thin film transistor. In this case, if the driving thin film transistor is an n-type, the driving thin film transistor is turned-on by applying a positive voltage to the gate.
Accordingly, the related art organic light-emitting display device has the following disadvantages. In the amorphous silicon thin film transistor, the threshold voltage is moved in the positive direction according to the positive stress of gate voltage, and the threshold voltage is moved in the negative direction according to the negative stress of gate voltage. The movement of the threshold voltage is not problematic if the duty ratio is small as shown in the LCD device. However, if the duty ratio is large as shown in the driving thin film transistor for the active-matrix type organic light-emitting display device, the movement of threshold voltage may affect the life span of the display.
In the related art organic light-emitting display device, four switching transistors are provided for one pixel. Thus, the portion provided with the switching elements and the storage capacitor serves as light-shielding portions, whereby the resolution and aperture ratio deteriorates, thereby lowering the picture quality.